The most important thing is to add the right amount of infill, it is needed as a support for top and bottom surfaces. Experienced users will know where and how to add layers of solid infill (leaving the rest with low or no infill at all), but for the rest, I suggest a 15% infill from start to end.
With a 0.4 nozzle, I printed our Holocube with 3 perimeters and 0.3 layer height. You can reduce layer height to improve quality.
Part number 2 needs a little support, check the images to know where is placed.
The pieces are designed with enough margin to fit together, but as you probably know each 3D printing has diferrent tolarances so... maybe you will have to sand a little bit.